Kalamazoo Amp Field Guide:
Converting an Amp to Use a Solid State Rectifier
DISCLAIMER:Tube amplifiers contain potentially lethal, high voltages even after they are unplugged, that may cause personal injury or death. Do not attempt to repair, modify, or work on any amplifier unless you are absolutely certain you know what you are doing.
NO GUARANTEEThese mods are all things I have tried, someone I know has tried, or are recommended by people who work on amplifiers for a living. Nevertheless, if you try any of these, you assume all responsibility for anything that happens, whether the amp explodes, you get zapped, or the amp suddenly increases in value because everyone falls in love with it. The glory, the pain, whatever, they're all yours. If you can't live with that, don't mess with the amp!
Why convert to a solid state rectifier? There are several potential (pun intended) reasons. To begin with, solid state diodes will probably outlast everything else in your amp. They run far cooler than tube rectifiers, and don't pull any heater current, so your amp will run quite a bit cooler. (The rectifier tube is almost always the hottest tube in any amp.)
According to popular belief (including at one time my own) this conversion will provide less sag, resulting in a punchier, cleaner sound until the volume is really cranked (far more with the Reverb 12 than the Model One or Model Two). In reality, one only experiences sag with push-pull amps biased in Class AB. The Model One and Two amps are Class A single-ended and not prone to sag. The Reverb 12 is Class A push-pull, also not prone to sag, and uses a solid state rectifier to begin with.
On the other hand, switching to solid state will push the voltages up slightly for a tad more headroom, but only by 5-10V; the 6X4 doesn't have as much voltage drop as most 5V rectifiers.
Finally, if your amp lacks punch, you may well find that simply replacing the rectifier tube with a new one restores the missing punch. Almost every Model One and Model Two I've seen that's been played a lot had a worn out 6X4.
If you like the way your amp starts distorting now, and the volume level this happens at, you may not want to do this conversion. Of course, you can always convert back by simply removing the diodes and replacing the rectifier tube! If you do, leave the standby switch in; it's still a good idea.
The following is verbatim from email by Justin Belshe, who first got me thinking about modding these amps. Thanks, Justin.
``Get a pair of 1N4007 diodes and solder the leads from the striped ends to pin 7 of the rectifier socket (remove the tube). Solder the other ends to pins 1 & 6.''
Some people add a standby switch with this mod to avoid cathode stripping by applying high voltage before the tubes are properly warmed up. There's a lot of debate now whether cathode stripping is a problem at the voltages prevalent in guitar amps. I'm now inclined to agree that they aren't necessary. At the same time, if you really want one, it certainly won't hurt anything so long as you pay attention to the unloaded cap voltages.
I would also add that you don't have to use 1N4007 diodes; anything from a 1N4005 (600 PIV, 1 amp) on up will work. But the higher the rating the better, and 1N4007s are cheap! Many people now like to use FREDs (fast recovery diodes) to avoid switching transients which can add perceived harshness to the amp's tones.
Thanks!Special thanks to Justin Belshe <JBelshe@AOL.com> (who in turn thanks Tim Swartz <TimTube@AOL.com>) for lots of ideas on rebuilding this amp.
Last updated: 28 October 2007
Copyright Y2K Miles O'Neal, Austin, TX. All rights reserved. Miles O'Neal <roadkills.r.us@XYZZY.gmail.com> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514